Conventional drive control for an assist motor used for an electric power steering device has a switching circuit having an H-bridge structure and controls to turn on and off the switching circuit to apply assist torque of rightward or leftward rotation to the assist motor and to assist a steering force applied to a steering wheel.
Specifically, FIG. 8 is a diagram showing a schematic structure of a conventional electric power steering controller. It is seen that a current to be applied from a battery BT1 to an assist motor M1 is applied from a different direction by switching between switches Q11 to Q14 forming an H-bridge structure, and the assist motor M1 is rotated rightward or leftward. For driving of the assist motor M1 rightward or leftward, steering torque of a steering wheel (not shown) is detected together with its polarity by a torque sensor 81, and based on the detected steering torque, an assist command signal generating portion 82 generates an assist command signal, and a switch controlling portion 84 controls to make switching including pulse width modulation (PWM) generated by a PWM signal generating portion according to the assist command signal.
FIG. 9 and FIG. 10 show flowcharts of the switches Q11 to Q14 by conventional switching control. For example, in FIG. 9, to rotate the assist motor M1 rightward, the switch Q14 is turned on and the switch Q11 is also turned on and off by a PWM signal having a duty corresponding to the assist command signal, and a current I81 is flowing only when the switch Q11 is turned on. And, to rotate the assist motor M1 leftward, the switch Q12 is turned on and the switch Q13 is also turned on and off by a PWM signal having a duty corresponding to the assist command signal, and when the switch Q13 is turned on, a current I82 opposite from the current I81 is flowing to the assist motor M1.
On the other hand, by the switching control shown in FIG. 10, the same PWM signal is applied to the switches Q11, Q14 on a line where the current I81 flows rightward to rotate the assist motor M1 rightward, and the same PWM signal is applied to the switches Q12, Q13 on a line for the current I82 corresponding to the leftward rotation to rotate the assist motor M1 leftward.
The electric power steering controller for controlling rightward and leftward rotations of an assist motor by the switching control of the H-bridge circuit is described in, for example, Japanese Patent Publication No. 2-32185 and Japanese Utility Model Application Laid-Open No. 63-112175.
However, such a conventional electric power steering controller has disadvantages that when an assist command signal is not present, all the switches Q11 to Q14 are turned off as the suspended states shown in FIG. 9 and FIG. 10, if an external force is applied through wheels or the like to rotate the assist motor, the assist motor is readily rotated rightward or leftward by the external force because the assist motor is in an open-circuit state, and the steering wheel cannot be controlled properly. And, straight driving of an automobile at high speed is degraded, and a driver is hard to operate the steering wheel.
And, even when the assist command signal is present, it is not different from the case that there is not the assist command signal substantially if the PWM signal is off. And, tires are easy to rotate due to an external force to keep the current wheel positions, and there are the same drawbacks as those described above. The assist command signal is a signal to flow a current including an electrifying polarity to the assist motor, and the PWM signal is generated based on this signal. And, absence of the assist command signal means that the assist command signal is zero.
On the other hand, in an electric power steering controller that the assist motor is a DC series-wound motor, a shunt resistor of the assist motor is fitted to the outside of the H-bridge circuit, the respective switches of the H-bridge circuit are power semiconductor switching elements such as MOSFET (MOS-type field-effect transistor) and IGBT (insulation gate type bipolar transistor), and a flywheel diode is connected in parallel therein by its p-n junction. When there is no assist command including a PWM signal-off state and an unexpected external force is applied to the assist motor, a regenerative current is generated by a coil in the assist motor and, when a closed circuit including the assist motor is formed, then, the regenerative current continues to circulate in the closed circuit and does not flow through the shunt resistor, so that the regenerative current actually flowing through the assist motor cannot be detected. Accordingly, even if an unexpected regenerative current is flowing through the assist motor when the external force is applied to the assist motor in a state that there is not an assist command, this regenerative current cannot be detected, and feedback control at high accuracy cannot be made. As a result, the assist motor cannot be controlled with high accuracy.